The present invention relates to a method of housing wafers mirror-polished, cleaned, dried of their surfaces and a container for housing the wafers, and a transfer process for housing the wafers into the container. Specifically, it relates to a method for housing wafers, a container for housing the wafers, and a transfer process for housing the wafers into the container, for the sake of decreasing the adhesion of particles generated in the process of transport and so forth on the surfaces of the mirror-polished silicon wafers.
The manufacturing process of semiconductor devices, for example, memory chips and LSI chips, is largely divided into following process steps: the single crystal producing step in which a single crystal silicon ingot is obtained from polycrystalline silicon; the wafer processing step in which the ingot is sliced and the thin plate-like wafers are mirror-polished; and the device manufacturing step in which various devices are fabricated on the mirror polished wafers. The each process step has various batch treatment divisions. Each of the treatments and steps is processed in batches or in other places, and with time intervals. Therefore, there is a case where the wafers are housed in containers to be stored or transported between the treatment divisions or steps.
Particularly, in almost all cases, process steps until mirror polishing step and the device manufacturing step in which various ICs are produced on the mirror polished wafers, are performed in different companies. In that case, the wafers processed until mirror polishing are housed in a container in a large number and shipped to a device manufacturing company. The container is often called shipping box.
As non-existence is required of particles, organic substances and ions on the surface of wafer in the device manufacturing step, cleaning for removal of the particles is performed at the last of the wafer processing step, that is, before housing the wafers into the container for shipment.
The influence of the shipping box was also examined. The result was that the chlorine and organic substances coming out from the box becomes problem, and improvements in the material, etc. of the box have been done. There is also disclosed a method in which the atmosphere in the box is substituted by nitrogen gas for the sake of preventing the generation of so-called xe2x80x9cKasumixe2x80x9d (cludiness) on the wafer(for example, Japanese Unexamined Patent Publication No. Hei 10-92918).
As described above, when wafers are shipped to a device manufacturing company, extreme cautions must be paid as to the protection of the surfaces of the wafers from contamination. But there has been no effective method for preventing the adhesion of particles to the surfaces of the wafers.
For example, an art is disclosed in Japanese Unexamined Patent Publication No. Hei 8-64666, in which wafers are housed in a gas-tight container filled with high purity steam obtained from a pure water bubbling section prepared at some midpoint in the supply route of dry gas, for the sake of removing the influence of particle and gas contamination.
The art is effective in the case where hydrofluoric acid treatment, which is referred to as hydrofluoric anhydride treatment in the Japanese Unexamined Patent Publication No. Hei 8-64666, is prepared at the next step. Also, the art is effective in reducing the quantity of high purity nitrogen gas for purging to increase atmosphere purity in the container.
However, according to the test result by the inventor of the present invention, it was proved that the art is not much useful in the prevention of adhesion of the particles generated from the container, and that it is not suitable as a housing method and transfer method for housing in the case the mirror-polished wafers are transported to a device manufacturing process.
When a number on thin plate-like wafers are housed and supported in a container for storage or transport, the container(shipping box) is subjected to various vibrations during transport. Scrubbing between the contacting parts of the box and falling off of the dust depositing on the inner surface of the box caused by the friction due to the vibrations, allow particles to be generated: these particles adhere to the wafers by the action of vibration and static electricity. Here particle is a collective name of fine grains of substance floating in space or adhering to the surface of a thing, and is composed of metals, inorganic substances, and organic substances.
The generation of particles in a container during transport has not been so problematic while the degree of integration of LSI is low and required design rules in LSI manufacturing process is not so minute. But the degree of integration has become higher, and for manufacturing LSI of 1 G level, particle size to be controlled has become small, making the increase of particles during transport not negligible problem. Finer design rules of sub-micron with increasing degree of integration has lowered the size of particle to be controlled from 0.16 xcexcm to 0.10 xcexcm. In the future generation of 0.13 xcexcm design rules, the particle size to be controlled will be 0.08 xcexcm.
Adherence of particles to the wafer in the device process causes not only a short or damage in the fine circuit, but the particle of metal and organic substance cause the degradation of device characteristics, inducing to low yields.
Therefore, these particles is listed in the inspection items, and the wafer transferred from the wafer processing step is inspected in the device process, in which acception or rejection check is done based on the size and number of particles.
Although the wafer is cleaned in the wafer processing step to remove the particles, many of device makers clean the wafer at receiving step of device process. Higher level of cleaning is necessary to clean the particles down to those of smaller size, and man-hour increases.
It is difficult to completely prevent the adhesion of particles generated during transport on wafers. Particularly, when the wafers are transported by a truck or air plane, the vibrations the wafers being subjected to are different according to the construction of the vehicle, road condition, handling of the container, etc., and so there are various methods for preventing the adhesion of particles on the wafers. But there has been no effective means for preventing the generation of particles in the container.
The object of the present invention is, in the light of the problems mentioned above, to provide a wafer housing method and container and transfer process for housing the wafer into the container, which allow the adhesion of particles to the minimum even if the particles are generated during transport.
The process of reaching the present invention will be described hereinbelow.
The inventor of the present invention found that, the adhesion of particles on the mirror polished wafers which are vacuum-dried after cleaning their surfaces and housed in a hermetically sealed container, is largely reduced in the case where the inside space of the container is kept dry(for example, relative humidity is 10% or lower, or 2 ppm or lower, preferably 1 ppm or lower in absolute humidity) and the container is vibrated, comparing to the case where the inside space of the container is kept highly wet(for example, relative humidity is 40%, or 6.5xcx9c8 ppm in absolute humidity) and the container is vibrated.
The reason that particles are reluctant to adhere to the surface of wafer in the air of low water content can be explained as follows: in the case the surface of wafer is dry without water, even if a particle accelerated by the vibration collides against the wafer, the particle is simply repelled at the surface and not adheres to the wafer, but in the case the air in the container is highly wet, the surface of the wafer is damped and the particle adheres to the surface by the action of the water on the surface.
The present invention was made based on the fact mentioned above. The wafer housing method according to the present invention is a method wherein wafers mirror-polished, cleaned, dried of their surfaces are stored in the hermetically sealed space of a container filled with highly clean, dry gas of relative humidity lower than or equal to 10%. To be more concrete, it proposes a wafer housing method wherein wafers mirror-polished, cleaned, dried of their surfaces are retained in a hermetically sealed space filled with highly clean, dry gas of which the vapor phase contacting the wafers is kept to absolute humidity of less than or equal to 2 ppm, preferably less than or equal to 1 ppm.
Further, the wafer housing container according to the present invention is wherein; the container is a hermetically sealed vessel for housing wafers mirror-polished, cleaned, dried of their surfaces; the gas in the container is dry gas; and the absolute humidity of the gas is less than or equal to 2 ppm, preferably less than or equal to 1 ppm.
Still further, the first aspect of the transfer method according to the present invention is a transfer method for housing wafers mirror-polished, cleaned, dried of their surfaces into a container body of which the top opening can be closed with hermeticity by a cover, wherein the method comprises;
a process of substituting the air in the container body for dry air by supplying the dry air from the top opening, positioning the container body in an attitude in which the top opening is directed in directions other than an upward direction;
a process of housing the wafers into the container body, letting the top opening face the wafer supply position and maintaining the substituted state by the dry air; and
a process of attaching the cover to the top opening after wafers are housed; and
the processes are performed under a highly clean, dry gas.
The second aspect of the transfer method according to the present invention is a transfer method for housing wafers mirror-polished, cleaned, dried of their surfaces into a container body of which the front opening can be closed with hermeticity by a cover, wherein the method comprises;
a process of substituting the air in the container body for dry air by supplying the dry air from the front opening, positioning the container body in an attitude in which the front opening is directed in directions other than an upward direction;
a process of housing the wafers into the container body, letting the front opening face the wafer supply position and maintaining the substituted state by the dry air; and
a process of attaching the cover to the top opening after wafers are housed; and
the processes are performed under a highly clean, dry gas.
Here, the highly clean, dry gas means, for example, highly clean, dry air containing water as little as possible and almost no particles and other impurities, to be more specific, a gas containing particles of 0.2 xcexcm or greater in grain size to less than or equal to 200 per cubic centimeter, preferably to less than or equal to 150 per cubic centimeter and its absolute humidity is lower than or equal to 2 ppm, preferably lower than or equal to 1 ppm of water.
The top opening of the container suitably disposed in an attitude in which the top opening is directed in directions other than an upward direction for effective exhausting the ordinary air (residual air containing water) in the container body to be substituted by the dry air, to be more specifically, in a downward or downward slanting direction. Further, the each step is preferably performed in a hermetically sealed space with absolute humidity of 2 ppm, preferably 1 ppm or lower.